In photographic enlargement and reproduction it is often desirable to alter the contrast of a photograph. Whereas reduction of contrast can readily be achieved by adding diffused light over the entire image during exposure of the photographic printing medium, the achievement of contrast enhancement is not as simple. The use of high contrast photographic printing paper usually results in graininess and loss of resolution. Complex procedures involving computer controlled point-by-point dissection and reconstruction of the image can achieve this result, but a method which will be within the budget of the commercial or hobby photographer is needed.
Methods of achieving contrast enhancement with photochromic glasses utilizing spatial filtering techniques are known. Thus U.S. Pat. No. 3,598,471 to Baldwin et al describes a method for processing the Fourier transform of an object utilizing a photochromic glass to achieve contrast enhancement thereof. However a multiple lens system is required for the utilization of this method, and contact printing cannot be accomplished therewith.
Many types of photochromic glasses are presently known, but the most widely available photochromic glasses are of the kind described in U.S. Pat. No. 3,208,860 to Armistead and Stookey which is the basic patent in the field. Such glasses may briefly be described as silicate glasses comprising submicroscopic crystals of a silver halide dispersed in the glass which are reversibly darkenable by the action of light. The properties of these and other photochromic glasses vary depending upon composition and the nature of the photochromic constituents; however, many glasses are known which are darkenable under the action of ultraviolet light and bleachable (returnable to the undarkened state) under the action of longer wavelength red or infrared light.
Glasses which are darkenable under the action of heat and bleachable by the action of light are also known, being described for example in U.S. Pat. No. 3,734,754 to Randall and Seward.